The lead-acid battery which is widely used as a starting battery for an automobile comprises a positive electrode holding lead dioxide, a negative electrode holding spongy lead, both held by a grid-like power collector made of lead or a lead alloy, and a separator inserted between electrodes for isolating them from each other, the electrodes and separator being held in a cell container filled with diluted sulfuric acid serving as the electrolyte.
Referring to the separator for a lead-acid battery, there are, for example, a separator composed of a sheet made from linter pulp in a wet process and cured with a phenolic resin and a glass mat bonded thereto, a separator composed of a sheet made from synthetic pulp, an inorganic powder and glass fibers in a wet process and a glass mat bonded thereto and a synthetic resin separator having ribs extrusion molded from a polyethylene resin and an inorganic powder.
The currently available separators for lead-acid batteries are mainly of the polyethylene resin type for several reasons, for example, (a) since the separator can be sealed at its both ends in a way forming an envelope by two pair of gears, thereby making it possible to assemble a battery quickly, and (b) since the projections in the shape of rib can be formed during resin molding, thereby eliminating the necessity for any expensive glass mat.
While the polyethylene resin separator has the advantages as stated above, it has essentially required a surface active agent improving its wetting property with the electrolyte, since it is mainly composed of a polyethylene resin. Particularly, with the polyethylene resin separator having projections in the shape of rib and no glass mat, the gas rising from the positive electrode in the overcharged state of the battery makes it relatively easy for the surface active agent to be eluted from the separator into the electrolyte and thereby cause the problems as stated below:    (1) The battery fails to have a predetermined capacity in its initial charged state, though rarely, since the surface active agent liberated or eluted from the separator acts as a reducing substance to inhibit the activation (oxidation-reduction reactions) of the positive and negative electrodes during the initial charging of the battery as the battery has its interior exposed to a high temperature and an oxidizing atmosphere during its initial charging (formation).    (2) As the lead-acid battery has an increasing tendency to be used in a high temperature and overcharged atmosphere, (a) the simultaneous occurrence of the evaporation of the electrolyte and the electrolysis of water brings about an increased loss of the electrolyte and makes the cell joints more likely to be exposed from the electrolyte, and (b) the reducing substance liberated or produced from the separator is changed by the oxidizing power in the battery into an organic acid having a lead solubility (for example, a volatile organic acid, such as acetic acid), and the organic acid causes corrosion in the battery and a reduction in cross-sectional area of the welded cell joints, resulting in a lower power collecting efficiency, and the battery has its high-efficiency discharge performance lowered, fails to produce a high-current discharge as required when starting the vehicle and tends to have a shortened life.